I lucked out and stumbled across a KSG Friday at my local gun dealer. I added a MagPul AFG angled Fore Grip to prevent short stroking and moved my Eotch holographic sight from my Benelli to the KSG. I want to also eventually add the choke adapter once they are available, but don't think they are yet. Please correct me if I am wrong.
My question is, what kind of iron sights, if any, do you think I should get?

Not sure on the choke but Carlson Comps makes a muzzle piece. It amazes me how some people are just stumbling into this gun. I have called every one within 100 miles of me and nothing. One gun shop had one for 30 seconds and poof sold along with a RFB to the same dude, LUCKY!

For those of you that have the backup sights, how do they work? They look like they fold down to use other optics like a reflex sight. Do you need to remove the other optic in order to use the backup sights? Is there a method for removing/reinstalling without the need to resight the reflex sight, etc.?

I have a Magpul BUIS rear sight on my AR. It folds down when I want to use my red dot sight and spings up and will co-witness with it (looking through the red dot) and the dot is just above the front sight post.

For those of you that have the backup sights, how do they work? They look like they fold down to use other optics like a reflex sight. Do you need to remove the other optic in order to use the backup sights? Is there a method for removing/reinstalling without the need to resight the reflex sight, etc.?

As DDelambre said, when co-witness with red dot sight you don't have to remove sight to use the iron sights. Just pop them up and your good to go. You can turn/leave the red dot on or off. It depends on what works for you.

I will throw my opinion out on this. It has been my experience that the top rail is to short for a decent sight radis. Futher the shot gun in general is not a percise tool by nature. The sights add weight and take time to deploy. I would stick with a good relex/red dot optic.